Light-emitting diodes (LEDs) are becoming more important to modern day life and it is envisaged that they will become one of the major applications in many forms of lighting such as automobile lights, traffic signals, general lighting, liquid crystal display (LCD) backlighting and display screens. Currently, LED devices are typically made from inorganic solid-state semiconductor materials. The material used to make the LED determines the color of light produced by the LED. Each material emits light with a particular wavelength spectrum, i.e., light having a particular mix of colors. Common materials include AlGaAs (red), AlGaInP (orange-yellow-green), and AlGaInN (green-blue).
LEDs that produce white light, which is a mixture of fundamental colors (e.g., red, green and blue) or that produce light not available using the usual LED semiconductor materials are needed for many applications. Currently the most usual method of color mixing to produce a required color, such as white, is to use a combination of phosphorescent materials that are placed on top of the solid-state LED whereby the light from the LED (the “primary light”) is absorbed by the phosphorescent material and then re-emitted at a different frequency (the “secondary light”). The phosphorescent material “down converts” a portion of the primary light.
Current phosphorescent materials used in down converting applications typically absorb UV or blue light and convert it to light having longer wavelengths, such as red or green light. A lighting device having a blue primary light source, such as a blue-emitting LED, combined with secondary phosphors that emit red and green light, can be used to produce white light.
The most common phosphor materials are solid-state semiconductor materials, such as trivalent rare-earth doped oxides or halophosphates. White emission can be obtained by blending a blue light-emitting LED with a green phosphor such as, SrGa2S4:Eu22+ and a red phosphor such as, SrSi5Ni8:Eu22+ or a UV light-emitting LED plus a yellow phosphor such as, Sr2P2O7:Eu2+; Mu2+, and a blue-green phosphor. White LEDs can also be made by combining a blue LED with a yellow phosphor.
Several problems are associated with solid-state down-converting phosphors. Color control and color rendering may be poor (i.e., color rendering index (CRI)<75), resulting in light that is unpleasant under many circumstances. Also, it is difficult to adjust the hue of emitted light; because the characteristic color emitted by any particular phosphor is a function of the material the phosphor is made of. If a suitable material does not exist, then certain hues may simply be unavailable. There is thus a need in the art for down-converting phosphors having greater flexibility and better color rendering than presently available.